Flutterby™! : Crushed

You know the "heat a beer can, put it upside down in water, watch it crush itself" science experiment? Yeah, like that, only done with a railroad tank car:

I was told the interior of the tank car was washed out & cleaned with steam. Then all the outlet valves were shut and the tank car was sealed. All the workers went home for the evening and when they returned, this is what they found.